You can find a very good synopsis of a recent study by Hamburg University into Internet usage habits over at WebSiteOptimization.com.

You should check out the clickthrough heatmap for Google (Figure 5: The Golden Triangle – Eye Tracking on Google Results (Hotchkiss 2005)) which shows the importance of the top 3 positions in Google SERPs.

The synopsis contains some other good reference material also.

If you are interested in the original Hamburg University study (not a bad read if you have the time and interest) you can view it here.

[...] The dataset contained 36,389,567 search queries with 19,434,540 clickthroughs. While we all knew the importance of the top 3 positions in the Google SERPs, this analysis further reinforces that fact: [...]